Measuring distances by calculating the transit time of a pulse acoustic wave from a transmitter to a reflective surface has been known in the art. Such measuring devices include ultrasonic transmitters which direct a beam of ultrasonic energy to a distant surface. The two way transit time for the ultrasonic burst to leave the transmitter and return is measured and converted to a distance measurement.
One of the difficulties in using this technique is that reflections from surfaces other than the one desired are received and are difficult to distinguish from the main reflection also white electrical noise present in the atmosphere from the surface of interest. This means that the received reflections include unwanted echoes making accurate measurements difficult. One of the reasons for this undesirable result is the spreading of the acoustic wave. The wave as it is transmitted becomes so wide that unwanted peripheral objects as well as objects in the direct line reflect a signal back to the sensor. Many attempts have been made to control this unwanted wide wave propagation and/or reflection.
U.S. Pat. No. 4,905,208 issued to Norman R. Dick shows a means of controlling the frequency of the output and the center frequency of the response to overcome this objection. U.S. Pat. No. 4,903,004 issued to Jeffrey W. Stark attempts to overcome this objection or limitation in the use of transducers to measure distance by utilizing a plurality of transducer units at the source and at the reflected surface of both. In addition U.S. Pat. Nos. 4,884,448, 4,551,722 and 4,731,762 have utilized a variety of schemes and apparatus to minimize these reflections from obstructions and thereby maximizing the utility of many ultrasonic devices.